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5/18/15 22.1 Earth’s Interior and Plate Tectonics Chapter 21: Planet Earth PHYSICAL SCIENCE - DOERFLER Key Ideas: ¡ How is Earth’s interior structured? ¡ How has the appearance of Earth changed over time? ¡ What geologic features are common near tectonic plate boundaries 22.1 Key Terms Crust What is Earth’s Interior Like Earth’s interior is made up of several distinct compositional layers Mantle Crust – topmost layer of Earth ¡ Made up of hard, solid rock ¡ Relatively thin and makes up only 1% of Earth’s total mass ¡ Crust beneath ocean is called oceanic crust (3.1 to 4.9 mi. thick ¡ Continental crust is thicker and less dense than oceanic crust Core Lithosphere Plate tectonics Subduction Fault ÷ Avg. thickness of 12 to 25 mi (deepest beneath high mountains, can be 70 km deep) Earth’s Interior Mantle – layer of rock beneath crust ¡ Denser than crust ¡ Nearly 1,800 mi thick ¡ Makes up nearly 80% of Earth’s total volume Scientists have never drilled to mantle Can only infer consistency from geologic events, like earthquakes and volcanoes Earth’s Interior Gets warmer with depth Caves are cool because the air and rock are shielded from the sun Travel deeper in to mine, temperatures rise South African gold mines ¡ Can be up to 2 mi deep ¡ Temperatures reach 120 degrees F Caused by energy from Earth’s interior 1 5/18/15 Earth’s Interior Mantle believed to be much thicker than crust Earth’s Core Composition is solid Nearly as hot as surface of the sun ¡ Believed to be anyways Outermost layer is rigid Core – center of the Earth, made of mainly iron and Going deeper a few hundred kilometers ¡ Mantle is extremely hot and under incredible pressure Geologists think the mantle is plastic ¡ Oft, and easily deformed like gum nickel Has two layers ¡ Inner core – solid metal ¡ Outer core – liquid metal surrounding inner core Earth’s Core Metals contained in core would boil on Earth’s surface Iron – boils at 4,982 degrees F Nickel – boils at 4,950 degrees F Pressure is so great on outer core form mantle and crust, substances in outer core can’t turn to gas form Similarly, atoms in inner core are pushed together due to pressure, remains solid at temps above boiling point Radioactivity and Internal Temperatures Plate Tectonics Interior contains radioactive isotopes 1900 – German scientists, Alfred Wegener, saw that Nuclei decay and release more energy ¡ Mainly uranium, thorium, and potassium eastern South America and western African coasts appeared to fit together Wegener hypothesized that all of the continents might have been part of one landmass in the past before they drifted apart Isotopes are quite rare ¡ Earth is so big, there is enough to provide huge amounts of energy as heat ¡ This energy is major factor in Earth’s high internal temperature ¡ Called this mass of land Pangaea 2 5/18/15 Plate Tectonics Wegener supported his idea with evidence ¡ Said 200 million years ago same kinds of animals lived on continents that are now oceans apart He argued that the animals could not have evolved on separate continents Fossil of Mesosaurus found in Brazil has also been found in western Africa Evidence of Wegener’s Wegener’s idea was called continental drift ¡ Theory that Earth’s surface is made up of large moving plates Idea was largely ignored until 1960s ¡ Discovered structures on ocean floors New technology provided images of “bands” ¡ Bands of rock have alternating magnetic polarities ¡ Bands differ from another in the magnetic alignment of the rocks that they contain Alignment of Oceanic Rocks Supports theory of moving plates Alignment of Oceanic Rock Molten rock pouring onto ocean floor ¡ Creates new rock and hydrothermal vents Evidence based on these aligned rocks (bands) ¡ Earth’s magnetic field has reversed many times in history ¡ North pole becomes south pole Iron minerals (magnetite) in magma align with On average, occurs once every 200,000 years Earth’s magnetic field Magnetic regions become fixed as it cools ¡ Creates permanent record of Earth’s magnetic field ¡ Bands are symmetrical on either side of Mid-Atlantic ridge Rocks are youngest near the ridge, older away from it ¡ Suggests the crust was moving away from plate boundary in both directions Earth Has Plates These plates move over mantle Lithosphere – approximately 100 km (60 mi) thick and made up of the crust and upper portion of the mantle ¡ Made up of 7 large pieces (and several smaller ones) called tectonic plates Continents are embedded into these plates ¡ Plates move in relation to one another ¡ This theory is called plate tectonics 3 5/18/15 Earth Has Plates How Do They Move? These plates move at a rate of 1 to 10 cm per year Scientists really don’t know how they do it May seem slow ¡ But they have moved considerable distances ¡ Have been doing this for millions of years Boundaries do not always coincide with continental boundaries Some plates move toward each other ¡ Some move away from each other How Do They Move? Plate Boundaries One hypothesis ¡ Suggests plate movement results from convection current in asthenosphere – hot, plastic portion of the mantle Plate tectonics helps scientists ¡ Study and predict volcanic eruptions ¡ Has provided information on earthquakes Plates of lithosphere float on asthenosphere ¡ Rock is “bubbling” like hot oatmeal and moves the plates slowly Volcanoes and earthquakes most often occur Others believe this method does not provide enough energy to move the plates ¡ They say that plates are driven by the force of gravity acting on their own massive weight where tectonic plates come together At plate boundaries, many other geologic features, such as mountains and rift valleys, can also occur Mid-Oceanic Ridges They are the result of divergent boundaries ¡ Occurs where two plates move apart and create a gap between them ¡ Hot rocks rise from the asthenosphere ¡ Cools to form lithosphere rock ¡ Two diverging plates pull the newly formed lithosphere from the gap ¡ Drop in pressure causes asthenosphere to partially melt, forming magma ¡ This then forms new oceanic crust Mid-Oceanic Ridges These are mountain ranges at divergent boundaries in oceanic crust ¡ Formed by magma rising to Earth’s surface and cooling Along center of mid-ocean ridge is rift valley ¡ Narrow valley that forms where plates separate Most studied is Mid-Atlantic Ridge ¡ Most of this ridge is under water ¡ A portion of it is at sea level in Iceland ¡ Run roughly down center of Atlantic Ocean, from Arctic Ocean to southern tip of South America 4 5/18/15 Oceanic Plates and Convergent Boundaries Oceanic plates dive beneath continental plates at convergent boundaries Newer lithosphere is created at divergent boundaries ¡ Older lithosphere is destroyed at convergent boundaries ¡ Example: Andes mountains in South America Subduction – denser lithosphere of oceanic plate sinks into the mantle, dragging oceanic crust with it Subduction Zones Subduction and Volcanoes Ocean trenches, mountains, and volcanoes are Chains of volcanic mountains form along overriding formed at subduction zones Ocean trenches: form along boundary between oceanic plates or oceanic and continental plates Water-bearing rock and sediment are heated by Can be very deep ¡ Marianas trench is deepest (deepest point is 6.8 mi beneath ocean surface) ¡ plate at subduction zones surrounding mantle Releases water into overlying mantle Water lowers melting point of rock at high pressures ¡ Magma forms and rises into crust ¡ ¡ Magma cools and low-density rocks build up over millions of years, creating high mountain chains and plateaus Islands Sometimes, convergent boundary occurs when two plates of oceanic lithospheres collide Denser of two plates sinks beneath other ¡ Forms deep oceanic trenches Fluids released from subducted plate causes mantle rock to melt, forming magma Magma rises, forming island arc ¡ Mountains Continental plates that collide produce mountain ranges ¡ Himalayas – tallest range in the world Grow in both width and height Since they move so slowly, they only grow several centimeters a year Chain of volcanic islands 5 5/18/15 Transform Fault Boundaries Plate movement can cause cracks in lithosphere Transform Fault Boundaries Transform fault boundary – when rock moves Once break occurs, rock continues to move and horizontally at faults along plate boundaries scrapes nearby rock Fault – crack where rock moves in this manner Plate movement at these areas is one cause of ¡ Can occur in any area where forces are great enough to break rock earthquakes ¡ San Andreas fault in California 21.2 What are Earthquakes You can see that earthquakes occur mostly at the boundaries of tectonic plates where the plates move with respect to one another Earthquakes occur at plate boundaries ¡ Energy is released as seismic waves Earthquakes and Waves Energy is transferred by waves ¡ Measured as shock waves Three types of waves are generated ¡ Longitudinal – originate from focus ÷ Move faster through rock than others waves to reach recording stations ÷ For these reasons, they are known as primary waves, or P waves ÷ Travel by compressing Earth’s crust in front of it and stretching the crust back of it Focus – exact point inside Earth where earthquake ÷ First originates Epicenter – point on surface directly above the focus ¡ ¡ Seismic waves move in all directions Damage here is usually greatest Transverse Wave Move more slowly than longitudinal waves These are called secondary or S waves Motion is similar to when is shaken up and down Surface Wave P and S waves spread out from focus like light leaving a light bulb Surface wave – moves only across Earth’s surface The surface bends and reshapes as it shakes Surface waves cause more destruction than P or S waves ¡ Act in a circular motion P and S shake buildings up and down and back and forth in high frequency ¡ Surface waves have rolling motion and longer frequncy 6 5/18/15 Measuring Earthquakes Because energy from earthquakes is transferred by waves, scientists can measure the waves to learn about earthquakes and about the interior of Earth through which the waves travel Scientists measure the power of each earthquake ¡ And the amount of damage the earthquake causes 7